In percutaneous transluminal coronary angioplasty (PTCA), a balloon catheter is inserted through a brachial or femoral artery, positioned across a coronary artery occlusion, and inflated to compress against atherosclerotic plaque to open, by remodeling, the lumen of the coronary artery. The balloon is then deflated and withdrawn. Problems with PTCA include formation of intimal flaps or torn arterial linings, both of which can create another occlusion in the lumen of the coronary artery. Moreover, thrombosis and restenosis may occur several months after the procedure and create a need for additional angioplasty or a surgical bypass operation. Stents are used to address these issues. Stents are small, intricate, implantable medical devices and are generally left implanted within the patient to reduce occlusions, inhibit thrombosis and restenosis, and maintain patency within vascular lumens such as, for example, the lumen of a coronary artery.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through an anatomical lumen to a desired treatment site, such as a lesion. “Deployment” corresponds to expansion of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into an anatomical lumen, advancing the catheter in the anatomical lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon prior to insertion in an anatomical lumen. At the treatment site within the lumen, the stent is expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn from the stent and the lumen, leaving the stent at the treatment site. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath. When the stent is at the treatment site, the sheath may be withdrawn which allows the stent to self-expand.
For example, FIG. 7 shows an upper portion of a stent 10 having an overall body shape that is hollow and tubular. The stent can be made from wires, fibers, coiled sheet, with or without gaps, or a scaffolding network of rings. The stent can have any particular geometrical configuration, such as a sinusoidal or serpentine strut configuration, and are not limited to what is illustrated in FIG. 7. The variation in strut patterns is virtually unlimited. The stent can be balloon expandable or self-expandable, both of which are well known in the art.
FIGS. 7 and 8 show stents with two different strut patterns. The stents are illustrated in an uncrimped or expanded state. In both FIGS. 7 and 8, the stent 10 includes many interconnecting struts 12, 14 separated from each other by gaps 16. The struts 12, 14 can be made of any suitable material, such as a biocompatible metal or polymer. The stent 10 has an overall longitudinal length 40 measured from opposite ends, referred to as the distal and proximal ends 22, 24. The stent 10 has an overall body 50 having a tube shape with a central passageway 17 passing through the entire longitudinal length of the stent. The central passageway has two circular openings, there being one circular opening at each of the distal and proximal ends 22, 24 of the overall tubular body 50. A central axis 18 runs through the central passageway in the center of the tubular body 50. At least some of the struts 12 are arranged in series to form sinusoidal or serpentine ring structures 20 that encircle the central axis 18.
FIG. 9 is an exemplary cross-sectional view of the stent 10 along line 9-9 in FIG. 8. There can be any number of struts 12, 14 along line 9-9. Line 9-9 runs perpendicular to the central axis 18 of the stent 10. In FIG. 9, the cross-section of seven struts 12, 14 are shown for ease of illustration. The struts 12, 14 in cross-section are arranged in a circular pattern having an outer diameter 26 and an inner diameter 28. The circular pattern encircles the central axis 18. A portion of the surface of each strut faces radially inward in a direction 30 facing toward the central axis 18. A portion of the surface of each strut faces radially outward in a direction 32 facing away from the central axis 18. The various strut surfaces that face radially outward collectively form the outer surface 34 of the stent 10. The various strut surfaces that face radially inward collectively form the inner surface 36 of the stent 10.
The terms “axial” and “longitudinal” are used interchangeably and relate to a direction, line or orientation that is parallel or substantially parallel to the central axis of a stent or a central axis of a cylindrical structure. The terms “circumferential” and “circumferentially” relate to a direction along a circumference of a stent or a circular structure. The terms “radial” and “radially” relate to a direction, line or orientation that is perpendicular or substantially perpendicular to the central axis of a stent or a central axis of a cylindrical structure.
Stents are often modified to provide drug delivery capabilities to further address thrombosis and restenosis. Stents may be coated with a polymeric carrier impregnated with a drug or therapeutic substance. A conventional method of coating includes applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
The stent must be able to satisfy a number of functional requirements. The stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel after deployment. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, is due to strength and rigidity around a circumferential direction of the stent. After deployment, the stent must also adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart.
In addition to high radial strength, the stent must also possess sufficient flexibility to allow for crimping on the a delivery device, flexure during delivery through an anatomical lumen, and expansion at the treatment site. Longitudinal flexibility is important to allow the stent to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. A stent should have sufficient toughness so that it is resistant to crack formation, particularly, in high strain regions during crimping, delivery, and deployment.
Furthermore, it may be desirable for a stent to be made of a biodegradable or bioerodable polymer. It is believed that biodegradable stents allow for improved healing of the anatomical lumen as compared to metal stents, which may lead to a reduced incidence of late stage thrombosis.
However, a potential shortcoming of polymer stents compared to metal stents of the same dimensions, is that polymer stents typically have less radial strength and rigidity. Relatively low radial strength potentially contributes to relatively high recoil of polymer stents after implantation into an anatomical lumen. “Recoil” refers to the undesired retraction of a stent radially inward from its deployed diameter due to radially compressive forces that bear upon it after deployment. Another potential problem with polymer stents is that struts can crack or fracture during crimping, delivery and deployment, especially for brittle polymers.
Some crystalline or semi-crystalline polymers that may be suitable for use in implantable medical devices generally have potential shortcomings with respect to some mechanical characteristics, in particular, fracture toughness, when used in stents. Some polymers, such as poly(L-lactide) (“PLLA”), are stiff and strong but can exhibit a brittle fracture mechanism at physiological conditions in which there is little or no plastic deformation prior to failure. A stent fabricated from such polymers can have insufficient toughness for the range of use of a stent. As a result, cracks, particularly in high strain regions, can be induced which can result in mechanical failure of the stent.
Stent performance may be measured in terms of the number of cracks or broken struts after crimping and deployment. Stent performance may be affected by complex interaction of many factors related to processing of the tubular construct out of which the strut pattern is formed, polymer material composition, polymer material morphology and microstructure, and the geometry and dimensions of the strut pattern itself. Factors related processing of the tubular construct include those associated with extrusion and subsequent blow molding as described in U.S. patent application Ser. No. 11/771,967, filed Jun. 29, 2007, “Method of Manufacturing a Stent from a Polymer Tube,” the contents of which are incorporated herein by reference. Processing factors that affect stent performance include without limitation draw down ratio during extrusion, blow molding temperature relative to glass transition temperature of the polymer, blow molding pressure used to expand the polymer tube, radial expansion ratio during blow molding, and axial extension during blow molding. These processing factors are used to modify the crystalline morphology and polymer chain orientation to achieve a desired combination of strength and fracture toughness along axial and radial directions.
There is a continuing need strut designs and manufacturing methods for fabricating polymeric stents that impart sufficient radial strength, fracture toughness, low recoil, and sufficient shape stability.